One way subterranean tools are actuated is to use the force of the hydrostatic pressure from well fluids at a specific location either alone or in conjunction with surface applied pressure, to set a tool. Typically this is done with an actuation piston exposed on one side to a low pressure and on the other side to wellbore hydrostatic acting on one side of a rupture disc. The piston is in pressure balance as long as the rupture disc is integral. At a predetermined depth the rupture disc is supposed to fail to expose one side of the piston to hydrostatic pressure. With the opposed side exposed to atmospheric pressure there is a net force on the piston which starts piston movement to actuate the tool.
Annular pistons have traditionally been used to actuate subterranean tools but more recently with the operating depths of tools increasing there has been a shift to the use of multiple rod pistons actuated with a breaking rupture disc. One such device is illustrated in U.S. Pat. No. 7,231,987. There are several issues with such designs. One is that large differential pressure from the hydrostatic pressure at deep locations and an atmospheric chamber at an opposite end of a rod piston tend to warp the piston bore from pressure differential. The other problem is that the rupture disc does not reliably burst at the desired pressure and in some applications can also fragment into small parts that can affect function of nearby components.
More recently a design was developed to keep rod pistons in pressure balance and actuate the piston with low pressure differential that is obtained by pressure buildup on opposed sides of the piston to keep the piston in pressure balance but at a predetermined time one side has a rupture disc that communicates with a low pressure chamber. As a result of the intermingling of high pressure with the atmospheric chamber there results a net force imbalance on the rod pistons and they are urged to move to set a subterranean tool such as a packer. A design that does this is shown in US 2014/0048281. The way it operates, shown in FIGS. 1-4 is the rupture disc 122 isolates a third chamber 124 from a first chamber 114 by closing off passage 148 between them. The second passage 115 is open through port 120 to annulus pressure. As the tool is lowered annulus pressure communicates through port 120 into the second chamber 115 and that communication raises the pressure in first chamber 114 without piston 112a movement. However as the pressure rises in the first chamber 114 the pressure differential on the rupture disc 122 rises while the third chamber pressure stays at a predetermined lower value such as atmospheric. At a predetermined differential across the rupture disc 122, it breaks to combine the first chamber 114 with the third chamber 124 to reduce the aggregate pressure to below the pressure in the second chamber 115 from annulus pressure through ports 120. The piston 112a moves in the direction of arrow A to set the packer 110.
While this design with a setting with low differential pressure was designed to address the issue of bore warping at high differential pressures for the rod pistons, it left unaddressed the issue of rupture discs and their variability of failure pressures that could result in tool setting at depths different than was intended. Other issues with rupture discs are that fragments can form upon burst that can disturb the operation of tools that are down the line. Another factor is the cost of the rupture discs and the assembly time and cost associated with mechanical assembly of the tool.
The present invention addresses these issues with elimination of the rupture disc while still allowing selective access between a high and lower pressure chamber on one side of the piston or pistons so that a differential pressure occurs to move the piston or pistons to set the tool. In the preferred embodiment the tool is a packer but other tools are contemplated. The pressure buildup against a spring biased piston allows for more predictable setting pressures for the tool in question. These and other aspects of the present invention will be more readily apparent to those skilled in the art from a review of the description of the preferred embodiment and the associated drawings while understanding that the full scope of the invention is to be determined by the appended claims.